It's looking pretty good, Muzzleflash. I like, in particular, the pretty good postures of the crew, reloading the Marder.
I'll make two comments, however. First, next time, trim your decals closer to the actual design. The way you have them, straight off the decal sheet, they show some silvering, which is common when you put decals onto a flat paint. If you still have a few spares, you could easily remove the crosses with some masking tape and re-apply new, better trimmed ones. It would improve the look of your model. For the other markings, you could try to paint over the 'silvered' areas..
Second, a building that is destroyed leaves a lot, I mean a very large amount of rubble... Your ruin looks too clean. There should be a lot of stuff at the bottom of the walls, both sides, and possibly a fairly large amount of dust everywhere else (unless it's an old ruin. The rubble is easy to do. Just glue some irregular bits of expanded polystyrene at the base of your walls (with wood glue!) then apply yet more wood glue over those bits and cover the lot with (clean) cat litter. Bits of broken up basla wood would add another touch of realism. You could airbrush that rubble with red brick tones and some black, or again just cover it with a rusty-coloured pastel dust. This dust will also cover some other areas of your base, grass and all and will make it look more realistic. You could dust the soldier boots and lower part of their uniforms with this dust too, as well as the lower parts of the vehicle (or at least the tracks).